The t(1;19)(q23,pl3.3) chromosomal translocation creates a fusion protein named E2A/pbxl (Kamps M P, et al., Cell 60:547-555, 1990; Nourse J, et al. Cell, 60:535-545, 1990). This protein is detected in 25% of childhood pre-B acute lymphoblastic leukemias (ALL) (Carroll A J, et al. Blood, 63:761-764, 1984; Williams D, et al. Cell, 36:101-109, 1984) and has also been reported to be present in some cases of adult ALL (Ohno H, et al. Internal Medicine, 32:584-587, 1993).
In normal B cells, the E2A gene encodes 3 transcription factors E12, E47, and ITF-1/E2-5 by alternative splicing of a common messenger RNA (Hanthom P, Science 1990; 247:467-470; Nelson C, et al. Genes Dev 4:1035-1043, 1990). These proteins belong to the basic helix-loop-helix (bHLH) family of transcription factors. It has been shown that this family of proteins forms homo and hetero dimers by interaction through their bHLH motifs. These complexes form tissue-specific transcriptional regulatory factors involved in various developmental pathways (Kadesch T., Immuno Today 13:31-36, 1992; Hu L Olson EN, et al. Mol Cell Biol, 12:1031-1042, 1992; Murre C, Biochem Biophys Acta, 1218:129-135, 1994). Pbx1 belongs to a homeobox gene family whose members are pbx1, pbx2, and pbx3. Pbx1 is expressed in all tissues except B- and T-cell lineages. The homeodomain of pbx1 binds to the sequence ATCAATCAA (VanDijk M A, et al. Proc Natl Acad Sci, USA 90:6061-6065, 1993; Ubrun D, et al. Oncogene 9:1641-1647, 1994; Lu Q Knoepfler P S, et al. Mol Cell Biol 15:3786-3795, 1995). Based on the high degree of homology of pbx1 homeodomain to yeast a1 and a2, two transcriptional repressors, it has been suggested that pbx1 might act similarly as a transcriptional repressor in mammalian cells (Kamps M P, et al., Cell 60:547-555, 1990; Nourse J, et al. Cell, 60:535-545, 1990).
The E2A/pbx1 fusion protein consists of an N-terminal portion of 483 amino acids of the E2A protein fused with 342 or 259 amino acids from the C-terminus of pbx1 (Kamps M P, et al., Cell 60:547-555, 1990; Nourse J, et al. Cell, 60:535-545, 1990). The different sizes of the pbx1 portion results from a splice variant of pbx1 which deletes a region of the pbx1 c-terminus to the homeobox domain. The junction in both forms is identical (Kamps M P, et al., Genes & Dev. 5:358-365, 1991). The bHLH and DNA binding domains of E2A are deleted as a result of the t(1;19) translocation, but the activation domain of E2A is still retained in the fusion protein. It has been shown that a reporter gene construct containing pbx1 homeodomain DNA binding sequence is transactivated by E2A/pbx1 but not by pbx1 (VanDijk M A, et al. Proc Natl Acad Sci, USA 90:6061-6065, 1993). This leads to the postulation that the spacially and temporally incorrect activation of pbx1 responsive genes contributes to the pre-B-ALL phenotype (VanDijk M A, et al. Proc Natl Acad Sci, USA 90:6061-6065, 1993; Ubrun D, et al. Oncogene 9:1641-1647, 1994; Lu Q Knoepfler P S, et al. Mol Cell Biol 15:3786-3795, 1995). Transgenic mice studies, however, show that the E2A region is essential and the pbx1 homeodomain is dispensable for the development of malignant lymphomas (Monica K, et al. Mol Cell Biol 14:8304-8314, 1994). This finding suggests that the oncogenesis might be due to interactions between cellular proteins with E2A/pbx1 instead of homeodomain-DNA interactions. Little is known about the oncogenic mechanism conferred by the E2A/pbx1 chimeric protein. Although leukemic patients carrying the t(1;19) translocation are associated with a poorer prognosis, it is not known how this genetic abnormality confers such aggressive behavior. Over 85% of ALL patients with t(1;19) expressing E2A/pbx1 transcripts exhibit the same breakpoint (Hunger S P, et al. Blood 77:687-692, 1991; Izraeli S, et al. Blood 80:1413-1417, 1992).
Berendes et al., Leukemia, 9:1321-1327, 1995, disclose a polyclonal antiserum which was generated using the fusion-point E2A-pbx1 peptide LSRPPDSYSYLSIR.
To date, attempts to raise monoclonal antibodies to the E2A/pbx1 fusion protein have failed to result in monoclonal antibodies immunospecific for the E2A/pbx1 fusion proteins alone. It has been found that an antigenic peptide spanning the junction region, which includes the amino acid sequence PDSYS (SEQ. ID. NO. 1), contributed from the E2A protein, does not result in the production of fusion specific monoclonal antibodies.